Hydraulic powered arm system with float control

ABSTRACT

The present invention relates to a hydraulically powered arm system with a hydraulic circuit, which hydraulic circuit (L) comprises a pressure source ( 6 ) and a lifting cylinder ( 1 ) arranged to an arm which is intended for handling a tool, said hydraulic circuit (L) comprising a partial circuit ( 45 A,  45 B), which can be connected into communication with said pressure cource ( 6 ) by means of a first valve element ( 44 ) and said lifting cyliner ( 1 ) by means of a second valve element ( 43 ), characterised in that said partial circuit ( 45 A,  45 B) comprises a pressure reducing/relieving unit ( 4 ) whichis arranged between said valve elements ( 43, 44 ), and that said pressure reducing/relieving unit ( 4 ) is controlled by a pressure controlling unit ( 62 A) in order to facilitate float control of the tool attached to the arm system, whereby a controlled ground pressure is obtained during the a floating motion.

The present invention relates to a hydraulically powered arm system witha hydraulic circuit, which hydraulic circuit comprises a pressure sourceand a lifting cylinder arranged to an arm which is intended for handlinga tool, said hydraulic circuit comprising a partial circuit, which canbe connected into communication with said pressure source by means of afirst valve element and said lifting cylinder by means of a second valveelement, in order to facilitate floating control of the arm system.

DESCRIPTION OF PRIOR ART AND PROBLEMS

Excavators, and similar machines, having a hydraulically powered armsystem, have the possibility to achieve a so called floating motion.During the floating motion, the tool (a bucket for an excavator) ispulled along the ground surface using the dead weight of the arm systemas the force by which the tool effects the ground surface. In case of anexcavator, this is achieved by means of allowing the lifting cylinder beconnected to tank, i.e. without being pressurized, and merely work withthe hydraulic cylinder between the boom and the stick. Theincontrollable ground pressure implies that the tool sometimes issubject to larger load than desired, which may lead to exaggerated wearand in the worst case to damages. Moreover, a floating motion that isperformed in this traditional manner provides a varying ground pressure,as the influence/moment of the dead weight varies with the position ofthe arm system. It is evident that in many cases it would be desirableto obtain substantially the same ground pressure along the entirefloating motion, which is normally performed in an inward direction.

BRIEF DISCLOSURE OF THE INVENTION

An object of the invention is to eliminate or at least minimize theabove mentioned drawbacks, which object is achieved by a hydraulicallypowered arm system with a hydraulic circuit, which hydraulic circuitcomprises a pressure source and a lifting cylinder arranged to an armwhich is intended for handling a tool, said hydraulic circuit comprisinga partial circuit, which can be connected into communication with saidpressure source by means of a first valve element and said liftingcylinder by means of a second valve element, wherein said partialcircuit comprises a pressure reducing/relieving unit, which is arrangedbetween said valve elements, and that a pressure controlling unit forcontrol of said pressure reducing/relieving unit in order to facilitatefloat control of the tool attached to the arm system, whereby acontrolled ground pressure is obtained during the a floating motion.

Thanks to the invention, a floating control may be achieved, where theground pressure may be chosen within a certain range, such that anundesired exaggerated load on the tool attached to the arm system may inprinciple be eliminated.

The invention is especially advantageous when used in connection with amobile handling device according PCT/SE99/01131, which discloses ahydraulic circuit comprising an accumulator system, by means of which aconsiderable amount of recovery of the energy for the lowering load maybe achieved, since the accumulator in such a circuit in principle makesit impossible to use traditional floating control.

According to further aspects of the invention:

-   -   said pressure reducing/relieving unit comprises a        reducing/relieving valve, which preferably is formed as an        integral unit,    -   said pressure controlling unit comprises a proportional valve,    -   said proportional valve receives signals directly or indirectly        from a control device handled by the operator, such that the        level of the pressure against the ground may be chosen within a        certain range,    -   said proportional valve is controlled by a control unit, which        besides signals from said control device also receives signals        from a position sensor, whereby an automatic compensation for        the position of the arm is achieved in order to obtain a        substantially constant level of the pressure against the ground        during a floating motion,    -   said pressure source comprises an accumulator,    -   said accumulator is included in a circuit for recovering and        recycling, respectively, of lowering load energy, the hydraulic        circuit also comprising a variable hydraulic machine with two        ports, said hydraulic machine being able to give full system        pressure in two flow directions to said ports, wherein one of        the ports is connected to said accumulator and the other port is        connected to said lifting cylinder.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described below more in detail with reference tothe enclosed drawing, in which a hydraulic circuit according to theinvention is schematically shown.

DETAILED DESCRIPTION

FIG. 1 shows a hydraulic scheme for a lifting cylinder in a hydrauliccircuit, which to essential parts coincides with what is shown inPCT/SE00/02360 and which is further supplemented according to theinvention. A double-acting hydraulic cylinder 1, a variablereciprocating pump 3 (which is called a hydraulic machine below) and anaccumulator assembly 6 are shown, which will be described more in detailbelow. The hydraulic circuit is disposed in an excavator, the liftingcylinder 1 thus being provided to carry out vertical work of the armwhich carries the bucket on the excavator. Disposed between the liftingcylinder 1 and the hydraulic machine 3 is a logic element 2, in the formof a stop valve, which is spring-loaded and which in its uninfluencedstate breaks the connection between the hydraulic machine 3 and thelifting cylinder 1. In its activated position, the valve device 2 givesopen communication between the hydraulic machine 3 and the liftingcylinder 1. (This logic element 2 also may function as a hose-rupturesafety device.) A similar logic element 5 is disposed between theaccumulator 6 and the hydraulic motor 3, with a function similar to thefirst-mentioned logic element 2. This too is in the form of a stop valve2. The operation of the hydraulic machine takes place in a manner knownper se via a suitable transmission, and preferably through a fuel-basedengine D.

The hydraulic machine 3 is a variable reciprocating pump which can bothreceive and emit oil at the ports 10, 11. The pump is of a known typewhich permits full system pressure at both outlet ports and in which theflow can be adjusted from zero to maximum by means of the variablesetting, which is normally achieved by means of a so-called swash plate.Using a pump of this kind eliminates the need to regulate the circuitvia a control valve, whereby a considerable simplification is achievedat the same time as control losses are reduced.

A safety valve 8 is provided in the system between the accumulator 6 anda tank 42, which ensures that a certain maximum pressure for the circuitis not exceeded. Moreover, there is a hose-rupture safety valve 61arranged in the circuit before the logic element 2.

A pressure-sensing element 17 is provided to register the pressure inthe line between the lifting cylinder 1 and the logic element 2. In theevent of a lowering motion which requires power, the pressure-sensingelement 17 will register that the pressure is below that required forthe function and ensure that oil is supplied to the bar side of thelifting cylinder. The purpose of the pressure-sensing element 17 is thusto ensure that the hydraulic machine 3 controls the flow down to zero,when the hydraulic cylinder no longer has any pressure, e.g. when thebucket has reached the ground level.

The system functions in principle such that in the event of a liftingmotion, the operator will send a control signal which will ensure thatthe valves 2 and 5 open. The connection between the accumulator 6,hydraulic machine 3 and lifting cylinder 1 is thus completely open. Thepressurized oil in the accumulator 6 flows then to the variablehydraulic machine 3, which conveys the oil onwards to the liftingcylinder 1. If the pressure in the accumulator in this case is higherthan that required to carry out the work using the lifting cylinder 1,the surplus energy may be supplied by the hydraulic machine 3 to thedrive system. If the accumulator pressure should not be quitesufficient, the variable hydraulic machine 3 provides a pressureincrease to reach the requisite pressure level, which is achieved bymeans of power, which is supplied via the engine D of the handlingmachine. Thus in such a situation only as much energy is supplied as isrequired to overcome the pressure difference between the accumulator andthe lifting cylinder's requirement. In the event of a lowering movement,the direction of flow in the pump is changed and oil is supplied at port10 and emitted at port 11 to be supplied to the accumulator 6. If thepressure in the accumulator 6 is then lower than at the lifting cylinder1, the variable hydraulic machine 3 will be able to supply energy. If onthe other hand the pressure in the accumulator is higher than in thelifting cylinder, additional energy from the engine D will need to besupplied to the variable hydraulic machine 3 to obtain a loweringmovement. However, this energy supplied is stored in the accumulator 6and is therefore accessible in connection with the next liftingmovement. It is evident from the above that the system is energy-savingand eliminates heat-generating throttling of the oil flow which normallyoccurs when the lowering energy is handled in conventional systems.

Further, a proportional valve 62 is shown, which allows small loweringmotions without utilizing the hydraulic machine 3, and which valve alsoincreases the capacity of the lowering motion when the hydraulic machinereaches its maximal capacity.

Further, the system is controlled by a control system 94, below calledthe computer 94, which suitably obtains information from sensors i.a.regarding pressure 91 and 92, respectively, position 90, and therotation speed of the engine.

When lowering the lifting cylinder, the major portion of the oil will bepumped to the accumulator system 6, but when the arm system suddenly isrelieved, when the bucket for instance hits the ground, the pressuresensor 17 in the lifting circuit must emit a signal to the computer 94to justify the pumping capacity downwards. During the transient time ofthe hydraulic machine, it must be supplied with oil in order not to bedestroyed (not to seize), and this amount is obtained from the refillingcircuit (20, 31A, and 31B, respectively), which instantly may providethe hydraulic machine 3 with oil during the transient time. Therefilling circuit comprises an accumulator 20 and non-return valves 31Aand 31B, respectively. Said accumulator 20 is preferably loaded by meansof a pump 120, which also may give an extra supply to the hydraulicmachine in case of a very long transient period, which otherwise couldlead to a situation where the amount of oil in the accumulator 20 wouldnot be sufficient.

The hydraulic machine chosen in the system has like all rotating pumps avolumetric loss, which at full flow and pressure may be expected toamount to 5% but at low flows it may be close on 100%, and said loss ofliquid must be replaced. It is important to realize that said loss ispractically independent of the deflection of the hydraulic machines orits flow. At a lowering motion, the entire amount of oil which isdelivered by the lifting cylinder will thus not be found in theaccumulator 6 but a portion thereof will run to the tank 42 via theleakage line of the hydraulic machine. Except said leakage,consideration must also be taken to the amount which is drained via thevalve 62. It must be possible to control the lowering motion of amachine with great accuracy, and the hydraulic machine 3 does then notgive sufficient control. For this reason, there is a valve 62 in thelowering circuit, which allows complete control. A lowering motion willtake place only via the valve 62 if small motions or great accuracy arerequired.

The hydraulic machine 3 has a size which allows full lifting speed, butit will be considerably more expensive to give the hydraulic machine asize which also manages full lowering speed, which is approximately 50%higher, i.e. which should require a flow which is approximately 50%higher. Further, this would imply a considerably extension of the lineareas etc. The valve 62 thus has two functions, partly to allow completecontrol at low lowering speeds, partly to increase the maximal loweringspeed at high lowering speeds. Or in other words, the valve 62 allowsthat a hydraulic machine 3 having considerably low capacity. Thiscontrol, the sequential control, is performed by the computer 94. Whenlow lowering speeds are desired, the computer does therefore not emitany signal to the hydraulic machine 3 or to the valve 2 and 6 but onlyto the valve 62. In this way, an exactly controlled motion withimmediate response is obtained. In this connection it may be pointed outthat the adjusting times of such a hydraulic machine 3 normally are felttoo long. When a higher lowering speed is desired, the computer emits asignal to the valves 2 and 5 to open while the hydraulic machine 3 isopened up. When a complete opening of the hydraulic machine 3 has beenachieved, the computer emits a signal to the proportional valve 62 toincrease the flow to the desired level. The maximal flow via theproportional valve is 50% of the pumping capacity.

In order to solve the problem which arises in connection with thefilling of the accumulator 6 with oil to ensure next lifting motion thefollowing details are added. The position sensor 90 of the liftingpiston 1 gives a signal to the computer 94, which also receives a signalfrom the accumulator system 6 by the pressure sensors 91/92. Then thecomputer 94 calculates the need and emits a signal to the pump 71, whichattends to desired/sufficient pressure being established, which in turndetermines the amount in the accumulator. Said refilling of theaccumulator is thus performed independent of a lowering motion orlifting motion being made or other functions being utilized. Thus, thecapacity of the pump 71 only has to be a fraction of the capacity of thehydraulic machine. The reason is that said refilling of the accumulator6 takes place during the entire operation period of the machine.

According to the invention, the hydraulic circuit L comprises a partialcircuit for automatic floating control of the bucket, which implies thatthe operator in an automatic manner with a in principle constant, chosenground pressure shall be able to move the bucket along the ground. Thispartial circuit includes a reducing/relieving valve 4, a secondproportional valve 62A, and two logic elements 43 and 44, respectively.The reducing/relieving valve 4, which is preferably formed by anintegral unit, comprises a type of slide, which in one direction isinfluenced by an adjustable (manually) resilient device 4B. Thereducing/relieving valve 4 is via a first line 45A and its logic element43 connected to a line between the hose-rupture safety device 61 and thefirst mentioned proportional valve 62. Thus, this line 45A is inconstant communication with the piston side of the lifting cylinder 1,under the condition that the hose-rupture safety valve 61 is open. Thereducing/relieving valve 4 is via a second line 45B and its logicelement 44 connected to a line between the accumulator 6 and the logicelement 5. Accordingly, this line 45B is constant communication with theaccumulator 6. The reducing/relieving valve 4 works in such a mannerthat it provides for a chosen pressure level independent of flowdirection therethrough. When the lifting cylinder 1 moves upwardly, onlya pressure reduction takes place, when the oil flows through the valve4, whereas in the opposition direction, when oil is evacuated out of thelifting cylinder 1, a relieving flow of oil out of reducing/relievingvalve 4 to the tank 42C takes place.

The proportional valve 62A is via a line 4A connected to thereducing/relieving valve 4 in such a manner that it provides for anadditional force that is controllable/adjustable acting together withthe resilient device 4B. The proportional valve 62A has an outlet thatleads to a tank 42B. This outlet is merely intended to handle theextremely small flows which are caused by the proportional valve 62A perse. Furthermore, there is an adjustable control device (not shown, e.g.a field regulator, an inductive gauge, a dielectric detector or thelike, which is suitably mounted on the switch panel), which may behandled by the operator in order control the level of the proportionalvalve 62A by means of the computer 94, which valve 62A in turndetermines the pressure level for the floating motion. Accordingly, if ahigh ground pressure is desired during the floating motion, the controldevice is set on a high level, whereas, if a low ground pressure isdesired, it will be set on a low level.

As already mentioned, a pressure sensor 17 and a position sensor 90(e.g. in-form of a field regulator, an inductive gauge, a dielectricdetector or the like) are provided on the lifting cylinder, whichsensors continuously emit the pressure and the position, respectively,of the lifting cylinder to the computer 94. By means of this lastmentioned information the computer 94 may approximately estimate theposition of the arm system, and hence also the moment the dead weight ofthe arm exerts on the lifting cylinder 1. As a consequence, it will bepossible to approximately calculate the static pressure, by means ofwhich the dead weight of the arm effects the pressure in the liftingcylinder 1, whereby the prerequisites for achieving continuous automaticfloating control are fulfilled. Since the pressure due to the deadweight continuously changes depending on the position of the arm, thisfactor has to be calculated if the ground pressure of the bucket shallbe kept in principally constant during the floating motion. This is alsocompensated for in an automatic manner by means of the computer 94continuously registrating the pressure and the position of the liftingcylinder.

When the operator desires an automatic floating control, he activates aspecial activator (not shown), e.g. a push button, which directly orindirectly emits a signal to the computer 94, whereby both of the logicunits 43, 44 open (the logic units 2, 5 of the basic circuit then haveto be closed), such that the reducing/relieving valve 4 via the lines45A, 45B is connected to the hydraulic system. By means of his controldevice the operator chooses the desired ground pressure for the floatingmotion. When the operator thereafter (during an inwardly directedmotion) emits an operation signal to pull the arm/bucket towards themachine an automatic adaptation of the pressure within the liftingcylinder 1 will take place, as the oil that flows from accumulator 6 isreduced to the desired pressure level during passage through thereducing/relieving valve 4, whereafter the oil that has been adapted tothe chosen pressure level (by means of the computer 94) flows into thelifting cylinder 1. During this motion, the position sensor 90 willcontinuously emit position signals to the computer 94, which therebyautomatically can compensate for changes of the moment as a consequenceof the dead weight, such that a substantially constant ground pressureis maintained during the whole floating motion inwards and towards theexcavator.

If instead, the floating motion is performed in the other direction,i.e. outwardly, the oil will have to be evacuated out of the liftingcylinder 1 via the line 45A. As the oil in this case cannot be allowedto flow into the accumulator 6, the oil entering into thereducing/relieving valve 4 has to be drained to the tank 42C, whichtakes place through line 4C. Also in this case it is the proportionalvalve 62A (controlled by the computer 94 which receives signals by theposition sensor 90 and by the operator's control device), which controlsthe reducing/relieving valve 4 to provide the desired pressure levelwithin the lifting cylinder 1.

In the computerized control system 94 there is preferably also anoptimal power output function included, in accordance with what isdisclosed in PCT/SE00/02360.

As is known, temperature variations may lead to operation problems. Itis not unlikely that a certain mobile handling device, e.g. anexcavator, at an occasion operates during heavy cold, wherein the gastemperature may be down to about −20° C., while the same handlingequipment at another occasion operates in environment with extreme heat,wherein the gas temperature may amount to about +70° C. Thus, it may bethe question of changes in temperature of almost 100° C. According to apreferred embodiment, a temperature sensor 95 is therefore provided,which sensor is connected to the gas phase within the accumulator 6. Bymeans of said temperature sensor 95 and the computer 94 the hydraulicpump 71 may then be controlled to give a loading pressure within theaccumulator 6 which is adapted to the gas temperature. The computer 94then registers and treat the signal form the sensor 95 first toestablish an optimal loading pressure depending on the temperature ofthe gas phase, and then to automatically cause the hydraulic pump 71 togive the desired loading pressure within the accumulator 6, i.e. about112 bars (102 bars+10%) at a gas temperature of −20° C., if thecalibration is set to 120 bars (+20° C.), which ensures that the systemoperates in a safe manner independent of the ambient temperature.

The invention is not limited to the above description but may variedwithin the scope of the appending claims. For example, it is evidentthat further position sensors may be included in the arm system, e.g.which also sense the position of the stick in relation to the boom, sothat moment from the dead weight of the arm may be monitored with stillgreater precision in order to achieve automatic floating control bymeans of reducing/relieving valve 4 within the hydraulic circuit.Furthermore, it is realized that the invention is not limited toexcavators but that it may be used in conjunction with all kinds ofhydraulic machines having an arm systems consisting of two (or possiblymore) parts, i.e. according to the principle boom/stick existing onexcavators. Moreover, it should be understood that the invention is notlimited to the above described integrated reducing/relieving valve 4 butthat the described function may be obtained by means of correspondingvalve parts which are not integrated, and that this kind offunctionality may be achieved by other types of valve elements which areinterconnected in order to achieve the same kind of function.Furthermore, it is evident that the pressure source may vary, e.g. to bein the form of a hydraulic pump instead of an accumulator 6.

1. A hydraulically powered arm system with a hydraulic circuit, whichhydraulic circuit (L) comprises a pressure source (6) and a liftingcylinder (1) arranged to an arm which is intended for handling a tool,said hydraulic circuit (L) comprising a partial circuit (45A, 45B),which can be connected into communication with said pressure source (6)by means of a first valve element (44) and said lifting cylinder (1) bymeans of a second valve element (43), characterised in that said partialcircuit (45A, 45B) comprises a pressure reducing/relieving unit (4)which is arranged between said valve elements (43, 44), and that saidpressure reducing/relieving unit (4) is controlled by a pressurecontrolling unit (62A) in order to facilitate float control of the toolattached to the arm system, whereby a controlled ground pressure isobtained during the floating motion.
 2. A system according to claim 1,characterised in that said pressure reducing/relieving unit (4)comprises a reducing/relieving valve (4), which is formed as an integralunit.
 3. A system according to claim 1, characterised in that saidpressure controlling unit (62A) comprises a proportional valve (62A). 4.A system according to claim 3, characterised in that said proportionalvalve (62A) receives signals directly or indirectly from a controldevice handled by the operator, such that the level of the pressureagainst the ground may be chosen within a certain range.
 5. A systemaccording to claim 4, characterised in that said proportional valve(62A) is controlled by a control unit (94), which besides signals fromsaid control device also receives signals from a position sensor (90),whereby an automatic compensation for the position of the arm isachieved in order to obtain a substantially constant level of thepressure against the ground during a floating motion.
 6. A systemaccording to claim 1, characterised in that said pressure source (6)comprises an accumulator (6).
 7. A system according to claim 6,characterised in that said accumulator (6) is included in a circuit forrecovering and recycling, respectively, of lowering load energy, thehydraulic circuit also comprising a variable hydraulic machine (3) withtwo ports (10, 11), said hydraulic machine being able to give fullsystem pressure in two flow directions to said ports, wherein one (11)of the ports is connected to said accumulator (6) and the other port(10) is connected to said lifting cylinder (1).